Widespread declines in biodiversity at both global and local scales have motivated considerable research directed toward understanding how changes in biological diversity may affect the stability and function of the ecosystems on which we rely. However, the research effort devoted to addressing this question in benthic systems has been minimal. In laboratory microcosms, we manipulated the number and composition of 3 functionally distinct benthic invertebrate freshwater species that are bioturbators of sediment over 3 biomass levels. Our objective was to test the effects of bioturbator diversity on rates and reliability of total dissolved P (TDP) flux between benthic and pelagic habitats. Both composition and species richness affected TDP flux. TDP flux was highest in the most species-rich community because of functional complementarity rather than selection effects. Furthermore, species richness enhanced TDP flux reliability by increasing the predictability of the biomass–TDP flux relationship by 30%, on average, for each species added. We attributed these nonadditive effects of invertebrate diversity to a combination of functionally mediated biogeochemical interactions and density-mediated interaction strength. Thus, our results suggest that bioturbator diversity can be important to nutrient cycling in aquatic ecosystems by strengthening benthic–pelagic coupling.